32 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



the surface. Once struck, however, a large one is 

 likely to put up an astonishingly active and 

 enduring resistance. We read, for example, of 

 one of 35 to 38 feet harpooned by Capt. N. E. 



Atwood off Provincetown, Mass., about 1863, 

 that towed the fishing smack all night, and broke 

 loose finally. 76 



'• Goode, Fish. Ind. U. S., 1884, Sect. 1, p. ( 



THRESHER SHARKS. FAMILY ALOPIIDAE 



The threshers (several species are known) are 

 peculiar among sharks for their enormously long 

 tail fin. Their closest affinities in other respects 

 are with the mackerel sharks. 



Thresher Alopias vulpinus (Bonnaterre) 1758 



Thraser; Swiveltail; Fox shark 



Bigelow and Schroeder, 1948, p. 167. 

 Garman, 1913, pi. 7, figs. 1-3. 



Description. — The thresher is as easily distin- 

 guished from all other Gulf of Maine sharks by its 

 long tail as the hammerhead is by its head, the 

 upper caudal lobe being a little longer than the 

 head and body of the fish together, curved much 

 like the blade of an ordinary scythe, and notched 

 near the tip, whereas the lower lobe measured along 

 the front margin is hardly longer than the pelvic 

 fins. We need merely point out in addition that 

 the first dorsal fin (of moderate size and about as 

 high as it is long) stands about midway between 

 pectoral and pelvic fins; that the second dorsal 

 fin and the anal are very small; that the pectoral 

 fin is long and sickle shaped; and that the 

 thresher is a stout-bodied shark with short snout 



and blunt, rounded nose. Its teeth are small, 

 subtriangular with a single sharp cusp and are 

 smooth edged. Those near the center of mouth 

 are nearly symmetrical, but the successive teeth 

 are increasingly oblique outward, with their outer 

 margins increasingly concave. 



Color. — Dark brown, blue-slate, slate gray, blue 

 gray, leaden or even nearly black above, often 

 with metallic luster, grading on the sides to white 

 below, except that the snout and the lower surface 

 of the pectorals are usually about as dark below 

 as above, and that the sides near the pectorals 

 may be more or less mottled with gray, the belly 

 also. The iris is black or green. 



Size. — Threshers vary considerably in size at 

 birth, for while free living specimens have been 

 reported as small as 46 inches, with manj r of 48 to 

 60 inches (some with umbilical scars still showing), 

 one unborn embryo was 61 inches long. The state 

 of development of the claspers of males, with the 

 lengths (14 ft. 6 in. and about 15K ft.) of females 

 that have been found with embryos, makes it 

 unlikely that they mature sexually until they are 

 at least 14 feet long (tail included). Lengths up 



-•.---' 



D 



^2 



Figure 9. — Thresher (Alopias vulpinus), about 5 feet- long, 

 Rhode Island, from Goode, drawing by H. L. Todd. 

 A, upper second tooth; B, upper third tooth; C, upper 

 fifth tooth; D, upper fifteenth tooth; E, lower second 

 tooth; F, lower sixth tooth, counted from center of 

 jaw; about 2 times natural size. From Bigelow and 

 Schroeder. Drawings by E. N. Fischer. 



